1. Field of the Invention
The present invention relates to a disc driving motor and an information recording and reproducing apparatus including the same. This disc driving motor can rotate a disc without runout by means of dynamic pressure of airflow generated by rotation of the disc, and is mainly used in a hard disc drive (HDD).
2. Description of the Related Art
In recent years, as for an HDD, there has been an increased demand for large storage capacity, high speed operation, low acoustic noise, low power consumption and the like. Hence, prime importance is placed on a technique for suppressing noise in a recording and reproducing head and a technique for stably rotating a disc.
An increase in storage capacity of the HDD is accelerated. As the storage capacity of the drive is increased, a track pitch on a disc becomes narrower. In order to trace the narrow track pitch with reliability, a motor of the HDD must rotate with high accuracy. In particular, an NRRO (Non Repeatable RunOut) property of a motor is indispensable to read a narrow track pitch on a rotating disc with reliability. This reading operation requires high accuracy on the order of sub-micron and, therefore, exerts a large influence on a highly dense storage capacity of an HDD.
Recently, HDDs tend to be installed in personal computers (PC) or audiovisual systems in order to record and reproduce moving images. In the case where an HDD records and reproduces moving images, it is essential that the access speed of the HDD is accelerated in order that a user of a PC or an audiovisual system comfortably plays back the moving images. In order to accelerate the access speed, it is necessary to rotate a motor at high speed.
In some cases, an HDD is reduced in size and thickness and operates a disc having a diameter of not more than 2.5 inches. Such an HDD is incorporated in a mobile device. In this case, it is assumed that a user uses the mobile device in a car or puts the mobile device in his/her pocket upon use. Therefore, it is necessary to improve resistance to external forces such as vibration of the HDD in order that the mobile device normally operates. Hence, the motor for driving the disc requires a resistance to external forces.
As a technique for suppressing noise, for example, U.S. Pat. No. 6,486,578 discloses a configuration that a magnetic shield plate is interposed between a motor unit and a disc (hereinafter, referred to as “conventional configuration 1”). FIG. 13 is an exploded perspective view showing a motor unit of an HDD in the conventional configuration 1. FIG. 14 is a sectional view showing the motor unit. As shown in FIGS. 13 and 14, a stator core 24 having a core winding 8 of plural phases wound there around is fixed onto a base 7. At an inner circumference of the stator core 24, a magnet 23 fixed to a hub 2 is rotatably supported by two ball bearings 21. Further, a magnetic shield plate 9 is fixed onto the stator core 24 and the core winding 8. With this configuration, it is possible to prevent magnetic leakage flux from the motor unit to a head 11 and to suppress noise superimposition with respect to a reproduction signal in the head 11.
As a technique for stably rotating a disc, for example, JP2001-076459A discloses the following configuration (hereinafter, referred to as “conventional configuration 2”). That is, an airflow guide plate is provided in the vicinity of a disc. Airflow is generated in the casing of an HDD by rotation of the disc and, then, is guided such that a pressure thereof is applied to a side face of a hub. Thus, a lateral pressure F is applied to a radial bearing in order to stably rotate the bearing. FIG. 15A is a plan view showing an HDD in the conventional configuration 2 and FIG. 15B is a sectional view showing the HDD. As shown in FIGS. 15A and 15B, an airflow guide plate 32 is interposed between adjacent discs 1. The airflow guide plate 32 has a guide face 32a located at a rear side in a disc rotating direction and formed into an arcuate shape in the vicinity of a hub 2. With this configuration, a rotational center of a rotor is decentered in a certain direction. Thus, it is possible to effectively prevent NRRO and RRO and to enhance rotation accuracy.
Further, JP2000-331460A discloses the following configuration (hereinafter, referred to as “conventional configuration 3”). That is, a squeeze air bearing plate partially having an annular smooth face expanded circumferentially and radially is fixedly provided on a disc so as to oppose a disc face with a clearance of not more than 0.3 mm provided there between; thus, flutter-vibration due to aerodynamic excitation of the disc is suppressed. FIG. 16 is a perspective view showing an HDD in the conventional configuration 3. Herein, in order to stably rotate a disc 1, a squeeze air bearing plate 33 is provided in the vicinity of the disc 1 so that disturbance of airflow around the disc 1 is suppressed. In addition, in order that the airflow becomes smooth around the squeeze air bearing plate 33, an inclined face 34 is formed at an end face of the squeeze air bearing plate 33 so that the end face of the squeeze air bearing plate 33 has a streamlined sectional shape.
In the conventional configuration 1, as shown in FIGS. 13 and 14, for a lead wire 22 from the core winding 8, the magnetic shield plate 9 includes a protrusion portion 9a. On the other hand, in the vicinity of a phase that the head 11 enters, the magnetic shield plate 9 includes a flat portion 9b so as to approach the base 7. It can be understood from the drawings in U.S. Pat. No. 6,486,578 that a step between the protrusion portion 9a and the flat portion 9b has a corner and steeply rises. In addition, an inner circumference portion of the magnetic shield plate 9 is located in the vicinity of the magnet 23, but steeply rises in the conventional configuration 1.
However, the aforementioned steep step causes the following problems. That is, since the step steeply rises, there is a possibility that a vortex 12 is generated at the corner of the step as shown in FIG. 5A. The vortex 12 disturbs a dynamic pressure generated between the disc 1 and the magnetic shield plate 9 to generate a disturbance, so that the dynamic pressure is unstably applied to the disc 1. Thus, there is a possibility that the behavior of the rotor is disturbed. This phenomenon occurs even if the rotational speed is only about 3000 rpm. As the rotational speed increases, the influence of this vortex becomes large.
When disturbance vibration is applied to the rotor including the disc 1, as shown in FIGS. 11A and 11B, relative angular deviation occurs between the head 11 and the disc 1. The head 11 is attached onto a flexible flexure 20, fixed to an actuator arm 19, through a gimbal mechanism (not shown). The head 11 floats by a floating height FH at a predetermined pitch angle θp and a predetermined roll angle θr relative to the disc 1. Herein, if the disc 1 vibrates, inclination of the surface of the disc 1 is disadvantageously changed in a pitch direction as denoted by “1a” or “1b” in FIG. 11A. Further, such inclination is disadvantageously changed in a roll direction as denoted by “1c” or “1d” in FIG. 11B. As a result, a posture of the head 11 relative to the disc 1 is changed disadvantageously, so that the floating height FH of the head 11 varies. Then, the head 11 comes into contact with the disc 1. Consequently, data recorded on the disc 1 is damaged and, in some cases, stiction occurs between lubricating oil (not shown) applied onto the disc 1 and the head 11, resulting in head crush. Thus, the HDD incurs critical damage. On the other hand, if the floating height FH is made sufficiently large in order to avoid the head crush, a magnetic gap between the head 11 and a magnetic layer on the disc 1 becomes large, resulting in insufficient head reproduction output. Consequently, the HDD incurs considerable restriction about its storage capacity for recordable data. Accordingly, it is necessary to prevent the disc 1 from vibration even when the HDD receives disturbance vibration.
In addition, the inner circumference portion of the magnetic shield plate 9, located in the vicinity of the magnet 23, also steeply rises. Therefore, if magnetic leakage flux from the magnet 23 is cross-linked to the magnetic shield plate 9, concentration of the flux readily occurs at the steeply risen portion and this leads to a disturbance magnetic field in the head 11. Consequently, acoustic noise is readily superimposed on a reproduction signal in the head 11. Further, the magnetic shield plate 9 made of a magnetic material generates cogging torque to thereby become a source of applying vibration to the rotor of the motor, leading to occurrence of NRRO and RRO. This phenomenon is conspicuous in a case of using a magnet having a large energy product, such as a neodymium-iron-boron magnet, for the purpose of achievement in low power consumption. In particular, an influence of the phenomenon becomes large in a case of using a sintered magnet rather than a resin magnet. A magnet having a large energy product generates not only cogging torque but also acoustic noise due to vibration of the magnetic shield plate 9.
In the conventional configuration 2, the disc 1 is sandwiched between the airflow guide plates 32, each having the guide face 32a, in the vertical direction. However, further size reduction is required for a small HDD having a size of not more than 1.8 inches. Hence, such an HDD can be equipped with only one disc. Accordingly, the disc cannot be sandwiched between the airflow guide plates 32. In the small HDD having a size of not more than 1.8 inches, the hub 2 protrudes from the disc 1 by not more than 1 mm and has a complex structure other than a simple cylindrical structure in some cases. Consequently, even when the airflow guide plate 32 is provided at the top face side of the disc 1, airflow is not sufficiently brought into contact with the hub 2, so that an airflow guide effect is not satisfactorily obtained in some cases. Further, in the small HDD having a size of not more than 1.8 inches, there is a space of about 1 mm between the bottom face of the disc 1 and magnetic shield plate 9 in many cases. Herein, in order to provide the airflow guide plate 32 on the magnetic shield plate 9, it is necessary to set the thickness of the airflow guide plate 32 at not more than 1 mm. It is difficult for such a thin airflow guide plate 32 to sufficiently guide airflow. As a result, the airflow guide plate 32 cannot exhibit a satisfactory effect in at least the HDD having a size of not more than 1.8 inches. Further, as shown in FIG. 15B, the guide face 32a of the airflow guide plate 32 is formed perpendicularly to the disc 1; therefore, vortexes are disadvantageously generated above and under the airflow guide plate 32. This vortex becomes a source of applying vibration to the disc, leading to occurrence of NRRO and RRO.
In the conventional configuration 3, the thickness of the squeeze air bearing plate 33 must be set at not more than 1 mm in the HDD having a size of not more than 1.8 inches, as in the conventional configuration 2. However, such a thin squeeze air bearing plate 33 is readily warped. The squeeze air bearing plate 33 fixedly provided on the magnetic shield plate 9 sometimes comes into contact with the bottom face of the disc 1 depending on accumulation of tolerances. Even when the squeeze air bearing plate 33 does not come into contact with the disc 1 in a state that the HDD having a size of not more than 1.8 inches receives no disturbance G, if the HDD is used for mobile application, a mobile device including the HDD receives large impact upon use in some cases. Herein, the disc 1 or the squeeze air bearing plate 33 is deformed, so that contact between the disc 1 and the squeeze air bearing plate 33 may occur. In order to prevent this contact, it is necessary to design the mobile device in sufficient consideration of height tolerance and flatness of the magnetic shield plate 9 and the squeeze air bearing plate 33. However, if a clearance is provided between the squeeze air bearing plate 33 and the disc 1 in order to prevent contact between the squeeze air bearing plate 33 and the disc 1, a squeeze air bearing effect cannot be obtained satisfactorily. In addition, if an HDD must be reduced in size and thickness as is the HDD having a size of not more than 1.8 inches, provision of the squeeze air bearing plate 33 above the disc 1 causes difficulty in allocation of an area for such provision. Further, a space above the disc 1 is limited; therefore the provision is actually difficult in view of variations in flatness and height tolerance of the squeeze air bearing plate 33.
The squeeze air bearing plate 33 has a complex shape and, therefore, is made of a resin or a material that can be subjected to pressing such as forging. In general, such a material has a low surface hardness and is readily damaged. In view of this disadvantage, the squeeze air bearing plate 33 includes the inclined face 34 such that airflow becomes smooth there around. Thus, the squeeze air bearing plate 33 can be sharpened. However, since the squeeze air bearing plate 33 is made of a relatively soft material as described above, the sharp end face is readily damaged. If the squeeze air bearing plate 33 is scratched, airflow is disturbed due to the scratch, so that a vortex is generated. As a result, flutter-vibration of the disc 1 increases and acoustic noise is generated. Accordingly, it is necessary to treat the squeeze air bearing plate 33 carefully or perform surface curing treatment such as Ni plating on the squeeze air bearing plate 33 upon manufacturing of the squeeze air bearing plate 33 or assembly of a drive. This leads to increase in cost.
Herein, it is considered that the squeeze air bearing plate 33 is provided under the disc 1. However, the HDD having a size of not more than 1.8 inches has a small allowance in a thickness direction; therefore, the squeeze air bearing plate 33 can not be interposed between the motor unit and the disc 1. Further, it is considered that a component similar in shape to the squeeze air bearing plate 33 is provided on the base 7. However, it is necessary to reduce an outer diameter of the motor unit because the disc 1 in the small-size HDD must have a small outer diameter in order to configure the squeeze air bearing plate on the base 7 at an outer circumference of the motor. This leads to an increase in power consumption in the motor unit since electromagnetic conversion efficiency of the motor deteriorates.